In the prior art rolling and overhead doors, one electric motor is generally used to selectively drive the take-up barrel and the take-down barrel in the case of a rolling door and a drum cable system in the case of overhead doors. In order to accomplish this task, an actuator is required to selectively drive each barrel or drum cable system for opening and closing a rolling or overhead door. Accordingly, the operator of the prior art device has many precision components which are not only bulky increasing the space requirements for installing such a device but also requiring a high degree of servicing increasing operating costs of such devices. Further, since the actuator remains engaged with either the take-up barrel or the take-down barrel, the device may not be operated manually in the event of a power loss. If the door cannot be opened or closed manually, entry of rescue personnel or the exit of trapped workers may be prevented.
U.S. Pat. No. 4,690,195, issued Sep. 1, 1987, discloses an operator for a rolling door which provides a power operator means for both rolling the door up and pulling the door downwards to the closing position. The rolling door is particularly useful in a medium pressure environment such as those found in mines.
In the second embodiment of U.S. Pat. No. 4,690,195, a single drive motor operates the take-up barrel and the take-down barrel by chain drive. The embodiment has found to be unsuitable as the components are required to be of high precision and accordingly have high replacement and servicing costs. Further, such system is slow and unresponsive as the engagement between parts is slow. Further, the sprockets being driven by the chain must be selected in accordance with the door height and the required door opening speed. Further, since the take-up barrel and the take-down barrel are driven at the same time, there are times when the door is either under-tensioned or over-tensioned during the travel since the diameter of the take-up barrel varies with the amount of door extended. Further, since one chain travels around at least three sprockets, the operator cannot be placed over the hood of the device thereby limiting the locations where the device can be installed.